As is well known in the art, organopolysiloxane polymers are prepared, for example, by polymerizing a cyclic organosiloxane in the presence of an alkaline catalyst, optionally neutralizing the catalyst for deactivation, and distilling off low volatiles from the reaction product. The degree of polymerization of such a polymer is controlled by the amount of a low molecular weight organopolysiloxane serving as a terminal stopper. Also the terminal unit structure of the resulting organopolysiloxane depends on the structure of the terminal stopper.
However, the above-described method for preparing organopolysiloxane has the drawback that a trace amount of water in the reactant can also function as the terminal stopper. The resulting organopolysiloxane has hydroxyl groups introduced into the terminal unit, departing from the desired terminal unit. When such a high molecular weight organopolysiloxane gum having terminal hydroxyl groups is mixed with a reinforcing agent such as silica to formulate a silicone rubber compound, the compound gives rise to a crepe hardening phenomenon with the lapse of time because the bonding reaction between hydroxyl groups at the end of organopolysiloxane gum and a silica surface is accelerated. Prior to use, the silicone rubber compound having undergone crepe hardening must be restored to the initial state by applying strong shear forces.
In the application where organopolysiloxane gum is used as a base component of a silicone rubber compound, it is desired to design the organopolysiloxane gum such that its terminal unit consists of a triorganosilyl group. Nevertheless, in the currently available gums, hydroxyl groups are introduced in the terminal unit owing to incidental factors as mentioned above. For the preparation of organopolysiloxane gum, it was needed to reduce the content of hydroxyl groups.
In this regard, the conventional method for preparing organopolysiloxane gum is generally preceded by a pretreatment for removing a trace amount of water from the starting reactants such as a cyclic organosiloxane and a low molecular weight, linear organopolysiloxane. A typical pretreatment is by blowing an inert gas such as nitrogen into the reactants or drying the reactants in the presence of a desiccant such as silica gel. However, such pretreatment adds to the number of steps and renders the overall process complicated.
One known means for reducing the influence of water in the reactants is disclosed in JP-A 58-69228 wherein part of cyclic organosiloxane is distilled off in the presence of a polymerization catalyst and at a temperature at least 10.degree. C. lower than the polymerization starting temperature. This method is also cumbersome because the complex pretreatment of the reactant is necessary.
Another technique of reducing hydroxyl groups in organopolysiloxane terminal units is by adding a triorganohalosilane and a hexaorganodisilazane for neutralizing the alkali catalyst as disclosed in JP-A 60-49033. This technique is successful in reducing hydroxyl groups, but raises the problem of metal equipment corrosion due to the use of halosilane.
Therefore, for the preparation of organopolysiloxane gum, it is desired to have a technique capable of effectively reducing the content of hydroxyl groups in the terminal unit.